This invention relates generally to microwave circuits and more particularly to testing fixtures with tuning capabilities for characterization of microwave devices.
As is known in the art a technique for characterizing an electrical device such as a field effect transistor is the scattering parameter testing technique involving measurement of transmission and reflection coefficients including VSWR and impedance characteristics. The transmission lines employed in testing are required to provide specific conditions, for example, 50 ohm lines for scattering parameters measurements. A second characterization technique, the so-called power/gain measurement technique, requires that the impedance of the transmission lines feeding energy to a characterizing device be matched to the impedance of such device. Such techniques therefore, may employ numerous test fixtures, for example, a straight through calibration, an RF short, a bias T section DC block capacitor (bias T/DC block) and tuning. For example, to measure scattering parameters an RF short reference calibration and a straight through calibration are required to compensate for contribution to the scattering parameter measurement of the characterizing device by the length of the transmission lines coupled thereto. Further, for power/gain measurements, since the impedance of the transmission line is matched to the input impedance of the characterizing device, some tuning of the circuits feeding such a device is required. In addition, a source of a bias signal in general applied to such device must be isolated from the RF signals by use of a bias T section DC/block capacitor, for example. Thus these separate tests for performance measurements or calibration procedures may involve separate hardware requiring numerous connection and disconnection operations with attendant loss and mismatch problems, recalibration requirements and extremely lengthy structures with high insertion loss for wide bandwidths of operation. For example, with a 2 GHz-18 GHz frequency range wavelengths may vary from the high end to the low end by a factor of 9 to 1. Hence, for accurate tuning of a microwave circuit, the width of a one quarter wavelength tuner may be so wide as to be prohibitive and make it impossible to cover a particular portion of the frequency end where wavelengths are very short. This introduces a requirement for separate structures with multiple tuners to encompass a wide frequency range. In addition to the foregoing it has become difficult to provide a unitary relatively compact microwave multipurpose circuit device package for testing, evaluating, calibrating, and tuning, as well as filtering, generating or amplifying of microwave energy over wide bandwidths.